HiWhen I bought my Verminator from BTAS they said always make sure the pellets are 5.52mm, JSB Exacts or AA Fields.I have tried a few other makes including Rangemaster & found the initial advice was correct, even in my FAC Gladiator anything smaller "just drops out of the end of the barrel" at a good 75fps less.So I would suggest you get the "Large" as shown in the picture.Better Luck next time.Chris